Adventures of an Aviatrix, in which a pilot travels the skies and the treacherous career path of Canadian commercial aviation, gaining knowledge and experience without losing her step, her licence, or her sense of humour.

Tuesday, July 05, 2011

When FL180 Doesn't Exist

I mentioned earlier an air traffic controller saying that FL180 didn't exist today, and that I would explain later. It's later now. Altimeters work based on air pressure changes with altitude, so they need a way to compensate for air pressure changes that are not associated with altitude change. So they have a calibration factor, called the QNH in Europe and the altimeter setting in North America, that you reset according to ATC instructions as you go from place to place. Once you get high enough that you are over all the mountains and short-range traffic, the benefit of adjusting for local changes in air pressure becomes less in comparison to the benefit of not having to keep changing your altimeter setting all the time, so above that altitude, the transition level, you use the standard altimeter setting everywhere.

In non-arctic Canada, the transition level is 18,000'/FL180. That means that we fly using the local altimeter setting all the way up to 18,000' and then we switch over to the standard altimeter setting of 29.92". There are plenty of airplanes flying IFR at 17000', using the local altimeter settings and if the local altimeter setting is low, an airplane flying at FL180, using 29.92 is not a thousand feet above the airplane flying at 17000', using the local altimeter setting. For every tenth of an inch below 29.92, that FL180 airplane is a hundred feet closer to the 17,000' one. But for safety there is supposed to be 1000' separation. So there's a rule.

When the altimeter setting is less than 29.92” Hg, there will be less than 1 000 feet vertical separation between an aircraft flying at 17 000 feet ASL with that altimeter setting and an aircraft flying at FL180, (with altimeter set at 29.92” Hg); therefore, the lowest usable flight level will be assigned or approved in accordance with the following table:

Altimeter Setting

Lowest Usable Flight Level

29.92” or higher

FL180

29.91” to 28.92”

FL190

28.91” to 27.92”

FL200

So on low pressure days, flight level 180 doesn't exist, but you won't be assigned 18,000' either. If you had a real operational need to fly at 18,000' when FL190 was the lowest avaialble flight level, I don't know how they'd handle it, probably with a block altitude, same as if you needed to fly between 17,000' and FL180 on a high pressure day.

10 comments:

Regarding the operational need to fly at 18,000 feet when the lowest available flight level is FL190 ... I guess it depends on how precise your altitude needs to be for your operation, but it's possible FL190 would work as it might actually be 18,000 feet, right? If, for example the sea level pressure was 28.92", that puts "FL0" at about 950 feet below MSL (1" mercury is about 950 feet IIRC ... though it changes with altitude). So this means that at a sea level pressure of 28.92", FL190 is about 18,050 feet if my calculation is correct. So if your operational need is 18,000 feet on a day with a pressure of 28.92" - you might just be able to ask for FL190.

Obviously that won't work so well if the setting/QNH is somewhere in between such as 29.5" which would put FL190 at about 18,500 feet, but it's worth thinking about.

Of course, using air pressure altimeters is always subject to slight variations as the local conditions change, more quickly than you can get updated altimeter settings from ground stations. I'm curious if aerial survey operations have more sophisticated ways of compensating for that.

Is there then a difference between the rules for flight level assignments and the transition altitude? Or phrasing the question another way, does the transition altitude remain at 18,000 Feet regardless the air pressure and its potential to alter the lowest available "flight level"?

Yes, the transition altitude is always the same in the same region in Canada. I don't know all the international rules. I also don't really know why different countries have different transition levels. I've put it on my list to research.

In the UK the transition altitude over most of the country is 3'000 ft. The exceptions are the London (around Heathrow) and Scottish (around Glasgow and Edinburgh) CTAs where it's 6'000 ft. We just don't have enough big hills that it's worth bothering with odd altimeter settings otherwise.

Snowdon in north Wales is over 3'000 ft but I think it's the only such in England and Wales. There are quite a few more in Scotland (mostly outside the Scottish CTA which mostly covers the lowlands).

Conversely, we do have the habit of flying around on QFE (altimeter zeroed at the airfield elevation) because there are few, if any, airfields in the country high enough to be off-scale doing this.

As Ed alluded to, my understanding of why different countries have different transition altitudes is often due to the height of the highest hills in that country.

The UK has no big mountains (comparatively) so the transition altitude is quite low.

New Zealand on the other hand has Mt Cook which is over 3,000m so its transition altitude of 13,000ft was set so that even if you're flying at a flight level and the atmospheric pressure is low (i.e. below 1013hPa which means that if for example 15,000ft is showing in your altimeter you'll actually be lower than that) you will still have sufficient clearance to not have to worry about running into Mt Cook should you be flying in cloud.

Interesting I just found out that the transtion altitude in NZ used to be 11,000ft but in 2004 they changed it to 13,000ft. (See here for the relevant amendment: www.caa.govt.nz/rules/Amendments/Part_091_041125_Amendment_12.pdf)

Now, I am enlightened. Thanks for this post. I've been wondering about this for a long time, and even though I could search-engine-of-your-preference something like this, there's still nothing like a story someone tells you.